class_docstr: 构建词汇表,增加词汇，删除低频词汇
docstr: 增加单个词汇
  Arg :
      word (String) : 增加的词汇
docstr_tok: ["\u589e\u52a0\u5355\u4e2a\u8bcd\u6c47","Arg",":","word","(","String",")",":","\u589e\u52a0\u7684\u8bcd\u6c47"]
code: def _add_word(self, word: str):
      """
      增加单个词汇
      Arg :
          word (String) : 增加的词汇
      """
      if word not in self.word2idx:
          self.word2idx[word] = self.count
          self.word2count[word] = 1
          self.idx2word[self.count] = word
          self.count += 1
      else:
          self.word2count[word] += 1
  
file_path: DeepKE/src/deepke/relation_extraction/standard/tools/vocab.py
imports: import logging
  from collections import OrderedDict
  from typing import Sequence, Optional
label_desc: Use image node_id 3 for calling the Vocab obj's underlying member method code with example usage: obj._add_word(word) without return types